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Enantiomers have almost all the same chemical and physical properties, so it can be tough to separate them. But it’s still super important that we know how to tell them apart! In this episode of Crash Course Organic Chemistry, we’ll recap all the types of isomers we’ve learned about so far, and also learn about polarimetry as a way to separate enantiomers and how to predict the number of stereoisomers a molecule will have.

Episode Sources:
“THINK BIG! Must the molecules of life always be Left-Handed or Right-Handed?” Smithsonian Magazine.
Spinoff 2004 - “A NATURAL WAY TO STAY SWEET”, NASA.

Series Sources:
Brown, W. H., Iverson, B. L., Ansyln, E. V., Foote, C., Organic Chemistry; 8th ed.; Cengage Learning, Boston, 2018.
Bruice, P. Y., Organic Chemistry, 7th ed.; Pearson Education, Inc., United States, 2014.
Clayden, J., Greeves, N., Warren., S., Organic Chemistry, 2nd ed.; Oxford University Press, New York, 2012.
Jones Jr., M.; Fleming, S. A., Organic Chemistry, 5th ed.; W. W. Norton & Company, New York, 2014.
Klein., D., Organic Chemistry; 1st ed.; John Wiley & Sons, United States, 2012.
Louden M., Organic Chemistry; 5th ed.; Roberts and Company Publishers, Colorado, 2009.
McMurry, J., Organic Chemistry, 9th ed.; Cengage Learning, Boston, 2016.
Smith, J. G., Organic chemistry; 6th ed.; McGraw-Hill Education, New York, 2020.
Wade., L. G., Organic Chemistry; 8th ed.; Pearson Education, Inc., United States, 2013.

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Hi! I’m Deboki Chakravarti and welcome back to Crash Course Organic Chemistry!

In episode 8, we discovered that some molecules had non-superimposable mirror images, called enantiomers. Enantiomers have almost all of the same chemical and physical properties, like the same melting and boiling points. So it’s tough to separate them.

But they interact differently in chiral environments, like how the receptors in our nose and on our tongues are pretty sensitive to chiral molecules. Two enantiomers might have very different smells and tastes. For instance, there’s (R)-carvone and (S)-carvone, also known by the mouthful of an IUPAC name 2-methyl-5-(prop-1-en-2-yl)cyclohex-2-en-1-one.

So… I’m gonna stick with carvone. To most people, (R)-carvone smells and tastes like spearmint, while (S)-carvone smells and tastes like caraway -- those earthy-tasting seeds that get sprinkled on top of rye bread. Now, I don’t know about you, but I don’t want to go around a chemistry lab tasting and smelling random chemicals to figure out what they are.

It sounds kinda dangerous. So organic chemists can tell enantiomers apart in other ways, like using light! [Theme music]. The light that surrounds us is generally moving (well, actually vibrating) in all directions.

But a slitted disc or lens can filter out some of it to leave behind light that’s vibrating in one direction, called plane-polarized light. Polarized sunglasses, for example, are designed to filter out the sunlight reflected off of horizontal surfaces like lakes, roadways, or even the hood of a car. These surfaces organize some of the scattered light that hits them, so it gets aligned with the flat surface when it bounces towards us.

Basically, some horizontal plane-polarized light gets reflected into our eyes. Polarized sunglass lenses have a special coating of molecules that line up to block a lot of this horizontal light. So there’s less glare from horizontal light reflecting off flat surfaces, but other light can pass through.

And if we hold two polarized sunglass lenses perpendicular to each other, the lenses appear to get very dark, because we’re blocking both horizontal and vertical light. By the way, this trick's not gonna work with any plain ol’ sunglasses you might have, because non-polarized lenses just generically filter out some light. But to bring all this back to enantiomers… chiral molecules also have cool interactions with plane-polarized light: a single enantiomer of the chiral molecule can rotate it left or right.

And using an instrument called a polarimeter, we can measure a molecule’s ability to turn light. A polarimeter has a light source and several plane-polarizing filters. There’s also a rotatable one called an analyzing filter, which is attached to a protractor to help us measure the angle of light.

A solution of an enantiomer is placed inside a sample chamber, which is sandwiched between the polarizing filters and the analyzing filter. We can turn the analyzing filter and observe how the light changes, until we reach a point where the most light possible is getting through (which will probably involve some twisting back-and-forth). Then, we read the angle on the protractor to see how much and in what direction the molecule rotated the light from the source.

Like I mentioned, enantiomers of chiral molecules can rotate the plane-polarized light either left or right. It’s a physical property of the molecule, like boiling point and melting point. Molecules that turn the light toward the left are called levorotatory, and are given the symbol L or minus.

Molecules that turn the light toward the right are called dextrorotatory, and are labeled D or plus. We saw this nomenclature with L- and D- glucose in episode 8! Because the amount that the light gets rotated depends on things like the amount of chiral molecules we have and properties of our polarimeter, we can calculate the specific rotation of a molecule using a fairly simple equation.

Alpha observed is the reading from the protractor on the analyzing filter, c is the sample concentration, and L is a property called the path length of the polarimeter. This gives the specific rotation, which is dependent on the wavelength of the light and temperature at which the experiment was performed. The tricky part is… there’s no easy rule to guess whether an R-enantiomer or an S-enantiomer will be levorotatory or dextrorotatory.

Polarimetry is an experimentally determined property, which means we have to stick enantiomers in a polarimeter to see how they rotate light. No shortcuts, sorry. For example, it was experimentally found that the (S)-enantiomer of carvone rotates plane-polarized light to the right.

To indicate that in its name, we can add a plus: (S)-(+)-carvone. The (R)-enantiomer was found to rotate plane-polarized light to the left, so it’s (R)-(-)-carvone. So I guess there’s one tiny shortcut here: opposite enantiomers will turn plane-polarized light the same amount in opposite directions.

When we only have one enantiomer in the sample chamber of a polarimeter, it’s called an optically pure sample. The light will rotate one way by a certain experimentally-determined amount. But if we mix an equal amount of (S)-(+)-carvone and (R)-(-)-carvone in the sample chamber, we have a racemic mixture.

The light rotations cancel each other out, so we’d observe no change in the angle of plane-polarized light. Sometimes chemical reactions make more of one enantiomer than another. So when that happens, we can use the known rotation of an optically pure sample and the observed rotation of whatever mixture we have to find the enantiomeric excess.

This tells us the percentage of each enantiomer in the mixture. Knowing what products we have is important because a set of enantiomers can have really different properties in chiral environments, like a chemical reaction, or as medicines in our bodies. One could be helpful, while another could be deadly.

Polarimetry can help keep us safe and informed. Not only that, but polarimetry is actually how we discovered stereochemistry and enantiomers in the first place… with a little help from wine. Let’s go to the Thought Bubble.

Sometimes, you might open a bottle of wine and find some crystals that grew on the cork or settled at the bottom. These crystals are salts derived from tartaric acid, which is commonly found in grapes. As early as 1832, we knew that tartaric acid salts could rotate plane-polarized light.

However in 1838, there were reports of a tartaric acid solution derived from the commercial production of the chemical that didn’t rotate plane-polarized light. These reports reached the desk of a young French biologist named Louis Pasteur. Yes, that Louis Pasteur who became known for pasteurization, bacteria, and other pivotal scientific ideas.

The idea that presumably similar solutions of tartaric acid salts both could and couldn’t rotate plane-polarized light was very bothersome to young Louis. So he got some of the powdered tartaric acid with no optical activity and he set about growing crystals by letting the solvent slowly evaporate, like growing rock candy on a string from a super concentrated solution of sugar and water. Then, using a microscope, Pasteur noticed that the salt crystals grew in two different shapes.

To be clear: a visible difference in enantiomer crystals is rare. It was mostly luck that he was studying tartaric acid! He separated the two different crystals using just a tweezer, made solutions out of both types, and found that the two solutions rotated plane-polarized light in equal, but opposite directions.

This was the first description of an isolated set of enantiomers! And this event is often considered the start of the study of stereochemistry. Thanks, Thought Bubble!

Fresh off our history lesson, it might be helpful to revisit all the isomers we’ve encountered so far. The first thing to remember is that isomers all have the same molecular formula. In Episode 6, we met conformational isomers when we looked at Newman projections.

One molecule can have many of these isomers, because they only differ in how the groups are arranged as we rotate around sigma bonds. Constitutional isomers have different connections between the same number and type of atoms. And stereoisomers have atoms connected in the same order but different spatial relationships between them.

Stereoisomers that have non-superimposable mirror images, like (S)-carvone and (R)-carvone, are enantiomers. We also have meso compounds, which are sort of in the same family of molecules. They have two or more chiral centers but aren’t chiral molecules because they have an internal plane of symmetry.

We’ll talk more about these in a minute... Finally, we have stereoisomers that are not mirror images at all, which are called diastereomers. We also know geometric isomers, like the cis- and trans- isomers around a double bond.

These can technically be classified as diastereomers, but we organic chemists more commonly refer to them as geometric isomers. Diastereomers can also be configurational diastereomers, which have the same atom layout but a different 3D space arrangement. In a pair of diastereomers, one or more of the chiral centers will be the same, but some chiral centers have to be different.

For example, two configurational diastereomers are (2R,4S)-4-bromopentan-2-ol and (2S,4S)- 4-bromopentan-2-ol. With all these different kinds of isomers, it might seem almost impossible to look at a molecule and figure out the maximum number of isomers it can have. But, thankfully, organic chemists have done centuries of hard work so we can sit back and use a simple formula.

And since we have wine on the mind, let’s use tartaric acid as our example. First, we need to count the number of chiral centers in our compound, which I’ll just call n for now. Tartaric acid, or 2,3-dihydroxybutanedioic acid, has two stereocenters!

So in tartaric acid, n equals 2. Then, we can use the formula 2 to the power of n to calculate the maximum number of stereoisomers. 2 to the power of 2 is 4, so tartaric acid should have 4 isomers max. Let’s draw them...

I think it’s easiest to start with the hydrogen atoms pointing away from me, so I’m already following the convention that the lowest priority group is pointing away, and I don’t have to do too much extra work. So let’s start with this isomer and assign the stereochemistry of these two chiral centers. First of all, this carbon is directly bonded to the oxygen atom of this alcohol, which has an atomic number of 8.

The other two groups are carbons with an atomic number of 6, so the alcohol is clearly the highest priority. Now, we need to choose the higher priority of the other two groups. Remember to think of the carboxylic acid carbon as being bonded to three oxygen atoms, which each have an atomic number of 8.

So the carboxylic acid has higher priority over the other carbon that’s bonded to only one oxygen atom and a lowly hydrogen with an atomic number of 1. When we draw our arc, it points counterclockwise. So we know this chiral center is S.

Now, let’s move over to the other chiral carbon. From highest to lowest, we have the oxygen of the alcohol, the carboxylic acid, and then the carbon with the hydrogen and alcohol attached. And of course the hydrogen atom that points into the screen is in dead last.

We can draw our arc, and see this chiral center is also S. So we get a name of (2S,3S)-tartaric acid. Next, let’s draw the mirror image of this compound.

If we use the same priorities to assign the stereochemistry, we get a name of (2R,3R)-tartaric acid. Then, for the third isomer we can show one alcohol group coming toward us and the other pointing away from us. And finally, the fourth isomer also has one alcohol group coming toward us and one pointing away from us -- just different ones.

But wait. If we look at those two isomers side by side… and then we rotate one around, we can overlap them. So the third and fourth isomers of tartaric acid are actually just one molecule!

This is because there’s an internal plane of symmetry that we can see if we rotate around this bond. Now the internal plane of symmetry is clear to see. It’s a meso compound!

And we can call it meso-tartaric acid. After all that investigation, we know that tartaric acid has three isomers. So we always need to think critically about the molecules we draw in organic chemistry.

Our 2 to the power of n formula helps us make sure we don’t miss any isomers, but we have to make sure that two aren’t actually the same thing! Stereochemistry plays an important role in how molecules interact with one another, from smells to medicines. So let’s try to keep all this in mind for the puzzles to come!

In this episode, we learned about:. Polarimetry and how enantiomers of chiral molecules rotate plane-polarized light in opposite directions. Measuring enantiomeric excess in an unequal mixture of enantiomers with a polarimeter.

That racemic mixtures contain equal amounts of enantiomers and don’t rotate plane-polarized light. And we did a recap of constitutional isomers, stereoisomers, enantiomers, and diastereomers. Next week, we’ll dive into water (the molecule, not an actual pool) to talk about polarity and how to show electrons moving around a structure.

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